/Engine/Source/Runtime/SlateCore/Public/Widgets/SWindow.h
For a given client size, calculate the window size required to accommodate any potential non-OS borders and title bars
[UE::Slate::FDeprecateVector2DResult](API\Runtime\SlateCore\Types\FDeprecateVector2DResult) GetWindowSizeFromClientSize ( [UE::Slate::FDeprecateVector2DParameter](API\Runtime\SlateCore\Types\FDeprecateVector2DParameter) InClientSize, [TOptional](API\Runtime\Core\IO\TOptional)< float > DPIScale )